1. Technical Field
This invention relates to printed wiring boards and more particularly to copper-clad insulating films for use in the wiring boards and also to a method of producing such board using such film by a subtractive process.
2. Prior Art
Printed wiring boards of a multi-layered type are known which are constructed with a plurality of electrically conductive circuits laminated by an insulating layer. These wiring boards have been applied to mechanized fabrication of electronic equipment and further to various other sectors of industry.
In general, the above wiring board is assembled by mounting copper patterns on two opposite surfaces of a dielectric base or substrate to thereby form a printed circuit board and subsequently laminating a copper foil over the circuit board on both sides with layers of a glass epoxy prepreg interposed. The prepreg is derived by impregnating an epoxy resin into fibrous glass. The copper foil is etched to provide printed circuitry. The circuit board is fabricated usually by a subtractive process from a copper-clad laminate commonly accepted in the industry.
Such known mode of assembly, however, is effected necessarily under elevated temperature and pressure conditions, say at 170.degree. C. and at 40 kg/cm.sup.2, so as to remove air entrained in the prepreg. This leaves the problem that the resulting laminate product will involve residual strains resulting from the heat and pressure while in lamination, the strains leading to pattern failure in the printed circuitry.